<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>212</button>
    <script>
       let btn = document.querySelector('button')
       btn.onclick = function(){
            alert('123')
       }
    // let a = +prompt()
    // let b = +prompt()
    // alert(a+b)
    </script>
</body>
</html>